1. Field of the Invention
The present invention relates to objective lenses (objectives) for endoscopes, and endoscopes. Particularly, the present invention relates to objective lenses for endoscopes that have both long back focus and wide angles of view, and endoscopes including the objective lenses for endoscopes.
2. Description of the Related Art
Conventionally, endoscopes were used in medical fields to observe the insides of patients' bodies and to treat the patients. Objective lenses for observation are arranged at the inserting ends of the endoscopes. In recent years, electronic endoscopes in which images obtained by using the objective lenses are imaged by solid-state imaging devices are generally used. As objective lenses appropriate for the electronic endoscopes, objective lenses disclosed, for example, in U.S. Patent Application Publication No. 20080249367, Japanese Unexamined Patent Publication No. 2008-152210, and Japanese Unexamined Patent Publication No. 2007-249189, which were invented by the inventor of the present invention, are well known.
Ordinarily, a single objective lens is used in observation by using an endoscope. Therefore, the same objective lens is used to observe the field of view during insertion of the endoscope into the body of a patient and to observe a diseased region in the body of the patient after stopping insertion of the endoscope. Hence, the objective lens of the endoscope needs to satisfy both of a demand that the objective lens should have a wide angle of view to make it easy to insert the endoscope into the body of a patient during insertion and a demand that the objective lens should magnify a diseased region at as high a magnification ratio as possible so that the diseased region is easily and accurately observed. Therefore, conventionally, objective lenses (hereinafter referred to as conventional-type objective lenses, or conventional type) in which large negative distortion was generated were mainly used to satisfy such demands. In the conventional-type objective lenses, the large negative distortion was generated to observe a central area of an image at a high magnification ratio and to observe a peripheral area of the image at a lower magnification ratio but with a wide angle of view so that a wide area is observed. Here, the “large negative distortion” generated in the conventional-type objective lens is represented by using a notation method in which ideal image height is ftanθ (f: focal length of entire system, θ: half angle of view), which is used to express the distortion of an imaging lens having a general angle of view.
Images obtained by using the conventional-type objective lens will be described with reference to FIGS. 32 and 33. FIG. 32 is a schematic diagram illustrating relationships between angles of view and image heights in an optical system of an equidistant projection method. FIG. 33 is a schematic diagram illustrating relationships between the angles of view and image heights in a conventional-type objective lens for an endoscope in a manner similar to FIG. 32. The equidistant projection method is widely adopted in fisheye lenses, which are wide-angle lenses. In the equidistant projection method, half angles of view and image heights are proportional to each other.
To simplify explanation, FIGS. 32 and 33 illustrate cases in which the maximum half angle of view is 90 degrees and the full angle of view is 180 degrees. Semicircles composed of a plurality of sectors illustrated in upper parts of FIGS. 32 and 33 represent angles of view. The center angles of the sectors indicate angles of view. Specifically, center line M is 0 degree, and an angle with respect to the center line M is a half angle of view. Further, in FIGS. 32 and 33, angles of view of 18 degrees, 36 degrees, 54 degrees, 72 degrees and 90 degrees, which are 20%, 40%, 60%, 80% and 100% of the maximum half angle of view respectively, are illustrated.
A plurality of concentric circles illustrated in the lower part of FIG. 32 represent image heights. In FIG. 32, circles C2, C4, C6, C8 and C10 represent image heights corresponding to half angles of view of 18 degrees, 36 degrees, 54 degrees, 72 degrees and 90 degrees, respectively. Circle C10, which is the outermost circle, represents the maximum image height. FIG. 32 illustrates the equidistant projection method, in which half angles of view and image heights are proportional to each other. Therefore, the circles C2, C4, C6, C8 and C10 are concentric circles that are equidistant from each other. When point A at a half angle of view that is 80% of the maximum half angle of view is projected onto an image field (image surface), point A′ is formed on the image field, and the image height of point A′ is 80% of maximum image height h10.
A plurality of concentric circles illustrated in the lower part of FIG. 33 represent image heights. In FIG. 33, circles Cj2, Cj4, Cj6, Cj8 and Cj10 represent image heights corresponding to half angles of view of 18 degrees, 36 degrees, 54 degrees, 72 degrees and 90 degrees, respectively. Circle Cj10, which is the outermost circle, represents the maximum image height. FIG. 33 illustrates the conventional type. Therefore, the circles Cj2, Cj4, Cj6, Cj8 and Cj10, which are concentric, are not equidistant from each other. In FIG. 33, a distance between the circle Cj2 and the circle Cj4 in the central area is wider than a distance between the circle Cj8 and the circle Cj10 in the peripheral area. When point B at a half angle of view that is 80% of the maximum half angle of view is projected onto an image field, point B′ is formed on the image field, and image height hj8 of point B′ is not 80% of maximum image height hj10 but greater than 80% of the maximum image height hj10.
When FIGS. 32 and 33 are compared with each other, in the conventional-type objective lens illustrated in FIG. 33, the ratio of an image of a region with a small angle of view is greater than the ratio of an image of a region with a large angle of view. Therefore, in FIG. 33, the image in the peripheral area of the image field looks compressed and small, compared with the image of the equidistant projection method illustrated in FIG. 32. In other words, in the conventional-type objective lens, the amount of data about the region with the large angle of view is smaller, compared with the equidistant projection method.
However, as the angles of view of lenses have become wider, if an object in the peripheral area of an image is observed too small, a risk of overlooking a lesion in the peripheral area increases. Therefore, it is not desirable that too large negative distortion is generated practically. Further, in recent years, the resolution (number of pixels) of solid-state imaging devices became higher, and the qualities of images obtained by using the solid-state imaging devices were improved. Therefore, a demand for obtaining more precise and accurate data also about the peripheral area of the image, which used to be regarded as being relatively less important, became higher.
Further, objective lenses for endoscopes need to have sufficiently long back focus. When a solid-state imaging device is mounted on an endoscope, and the image field of the solid-state imaging device is arranged parallel to the long axis of the insertion portion of the endoscope, an optical path conversion prism for bending an optical path is provided between the objective lens and the solid-state imaging device. Therefore, it is necessary that a distance between the end surface of the objective lens and the imaging position is sufficiently long to insert the optical path conversion prism therebetween (a distance substantially similar to the back focus).